1. Field of the Invention
Intake systems for multi-cylinder internal combustion engines conventionally have an intake manifold which comprises individual intake passage pipes, each leading to respective cylinders and connecting, via a common intake passage pipe, a carburetor and the respective cylinders. Arranging and joining upstream end portions of the individual intake pipes in a regular polygonal formation makes it easy to deliver a fuel mixture from the single carburetor substantially equally into the respective cylinders and contributes to making the intake manifold compact in overall size. It also contributes to reducing vibrations and noises of the multi-cylinder internal combustion engine. Such an intake system is known from, for instance, Japanese Unexamined Utility Model Publication No. 57-101367.
2. Description of Related Art
Intake systems of this kind, for use with the fuel injection type of multi-cylinder internal combustion engines, have a fuel injector (i.e., a downstream fuel injector) after a throttle valve in a downstream portion of each individual intake passage pipe and one or two fuel injectors (i.e., upstream fuel injectors) in a downstream portion of the common intake passage pipe adjacent to upstream ends of the individual intake passage pipes. If the upstream fuel injector is installed in the downstream portion of the common intake passage pipe adjacent to the upstream ends of the individual intake passage pipes, which are symmetrically joined together at their upstream end portions, it is hard to make the intake manifold compact in overall size, and a uniform distribution of fuel mixture into the cylinders is difficult to achieve.